Projection exposure method, system and objective

ABSTRACT

A projection exposure method includes exposing an exposure area of a radiation sensitive substrate with at least one image of a pattern of a mask in a scanning operation. The scanning operation includes moving the mask relative to an effective object field of the projection objective and simultaneously moving the substrate relative to an effective image field of the projection objective in respective scanning directions. The projection exposure method also includes changing imaging properties of the projection objective actively during the scanning operation according to a given time profile to change dynamically at least one aberration of the projection objective between a beginning and an end of the scanning operation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of, and claims priority under 35 USC120 to, U.S. application Ser. No. 12/699,529, filed Feb. 3, 2010, nowU.S. Pat. No. 8,873,022, which claims priority under 35 U.S.C. §119 toEuropean Patent Application EP 09001938.1, filed Feb. 12, 2009. Thecontents of these applications are hereby incorporated by reference intheir entirety.

FIELD

The disclosure relates to a projection exposure method for exposing aradiation-sensitive substrate, arranged in the region of an imagesurface of a projection objective, with at least one image of a patternof a mask arranged in the region of an object surface of the projectionobjective. The disclosure further relates to a projection exposuresystem suitable for carrying out the method, and to a projectionobjective suitable to be used in such a projection exposure system.

BACKGROUND

Microlithographic projection exposure methods and systems are currentlyused to fabricate semiconductor components and other finely patternedcomponents. A microlithographic exposure process involves using a mask(reticle) that carries or forms a pattern of a structure to be imaged.The pattern is positioned in a projection exposure system between anillumination system and a projection objective in a region of the objectsurface of the projection objective. Primary radiation is provided by aprimary radiation source and transformed by optical components of theillumination system to produce illumination radiation directed at thepattern of the mask in an illuminated field. The radiation modified bythe mask and the pattern passes through the projection objective, whichforms an image of the pattern in the image surface of the projectionobjective, where a substrate to be exposed is arranged. The substratenormally carries a radiation-sensitive layer (photoresist).

When a microlithographic projection exposure system is used in themanufacture of integrated circuits, the mask (reticle) may contain acircuit pattern corresponding to an individual layer of the integratedcircuit. This pattern can be imaged onto an exposure area on asemiconductor wafer which serves as a substrate. The exposure area issometimes referred to as a die. A die in the context of integratedcircuits is a small block of semiconducting material, on which a givenfunctional circuit is fabricated. A single wafer typically contains alarge number of adjacent dies which are successively exposed to an imageof the pattern.

In one class of microlithographic projection exposure systems each dieis irradiated by exposing the entire pattern of the reticle onto the dieat once. Such apparatuses are commonly referred to as wafer steppers.

In alternative exposure systems commonly referred to as step-and-scanapparatus or wafer scanner, each exposure area is irradiatedprogressively in a scanning operation by moving the mask relative to anillumination beam in an effective object field of the projectionobjective, and simultaneously moving the substrate relative to theprojection beam in the conjugate effective image field of the projectionobjective in respective scanning directions. The mask is typically heldin place by a mask holder, which is movable parallel to the objectsurface of the projection objective in a scanning apparatus. Thesubstrate is typically held by a substrate holder, which is movableparallel to the image surface in a scanning apparatus. The scanningdirections may be parallel to each other or anti-parallel to each other,for example. During the scanning operation, the speed of movement of themask and the speed of movement of the substrate are interrelated via themagnification ratio β of the projection objective, which is smaller than1 for reduction projection objectives.

Forming a faithful image of a pattern on the substrate with sufficientcontrast typically involves the substrate surface should lying in thefocal region of the projection objective during exposure. Morespecifically, the substrate surface should be arranged in the region ofthe depth of focus (DOF) of the projection objective, which isproportional to the Rayleigh unit RU defined as RU=λ/NA², where X is theoperating wavelength of the projection exposure system and NA is theimage-side numerical aperture of the projection objective. Deepultraviolet (DUV) lithography λ=193 nm, for example, typically involvesa projection objective with a numerical aperture of 0.75 or higher toachieve 0.2 μm or smaller features. In this NA-region, the depth offocus is typically some tenth of a micrometer. In general, the depth offocus tends to decrease as the resolving power of the projection systemis increased.

It has long been recognized that systems having relatively narrow depthof focus may involve special technical measures to ensure that theexposure area on the substrate is in focus during exposure.

Imaging errors may also be introduced as a result of gravity falsescausing a mask shape to deviate from a planar shape. This effect isfrequently referred to as “reticle bending”.

SUMMARY

In some embodiments, the disclosure provides a step-and-scan projectionexposure method which allows for high quality imaging on substrateshaving an uneven substrate surface.

In certain embodiments, the disclosure provides a step-and-scanprojection exposure method which allows high quality imaging in caseswhere a pattern to be imaged is formed on an uneven mask surface.

In some embodiments, the disclosure provides a projection exposuremethod that includes exposing an exposure area of a radiation-sensitivesubstrate arranged in an image surface of a projection objective with atleast one image of a pattern of a mask arranged in an object surface ofthe projection objective in a scanning operation. The scanning operationincludes moving the mask relative to an effective object field of theprojection objective and simultaneously moving the substrafe relative toan effective image field of the projection objective in respectivescanning directions. The projection exposure method also includeschanging imaging properties of the projection objective actively duringthe scanning operation according to a given time profile to changedynamically at least one aberration of the projection objective betweena beginning and an end of the scanning operation. Changing at least oneimaging property of the projection objective includes changing in aspatially resolving manner an optical effect caused by at least onefield element of the projection objective. The field element includes atleast one optical surface arranged in a projection beam path opticallyclose to a field surface of the projection objective. At least one fieldelement is a mirror having a reflective surface arranged in theprojection beam path optically close to a field surface. Changing atleast one imaging property of the projection objective includes changingoptical properties of the mirror by changing a surface profile of thereflective surface of the mirror in an optically used area.

The inventors have recognized that conventional methods to ensure afocused imaging of fine structures onto a substrate may not besufficient in all cases to ensure a high yield of properly exposedsubstrates and thereby to reduce the reject rate. Specifically, it hasbeen recognized that the reject rate may significantly be influenced bya local unevenness (unflatness) of the substrate surface within theexposure area. If the relative position of the substrate surface withinthe exposure area varies across the exposure area in a magnitude beyondan acceptable range, parts of the pattern to be imaged within theexposure are may not be sufficiently defined in the structuredcomponent, thereby increasing the probability of component failureduring the lifetime of the component.

The potential problem may be exemplified when considering, for example,that the depth of focus of projection systems typically designed toprint images at the 45 nm node will typically have a depth of focus inthe range between about 100 nm and about 150 nm Specifications publishedin the INTERNATIONAL TECHNOLOGY ROAD MAP FOR SEMICONDUCTORS: 2006Update, Table 67a indicate that typical desired properties for the siteflatness SFQR of wafer substrates within a 26 mm×8 mm exposure area of ascanner system may correspond to the associated DRAM ½ pitch value indirect random access memories (DRAM). For example, those specificationsallow using wafers having a site flatness of up to 45 nm within theexposure area of a scanner when printing structures characterized by aDRAM ½ pitch of 45 nm. It is contemplated that conventional measures foraccurately placing a substrate surface within the focus region of theprojection objective may not be sufficient to account for those valuesof allowable unevenness of the substrate surfaces within the exposurearea in future scanning systems.

In some projection exposure methods, an active manipulation of theimaging properties of the projection objective is performed during thescanning operation, i.e. during the time interval between the beginningof one single scan and the end of one single scan, in which thesubstrate is moved relative to the effective image field of theprojection objective to successively print parts of the pattern of themask onto the substrate. The active manipulation causes at least oneaberration property of the projection objective to be varied dynamicallyin a targeted fashion according to a given time profile during the scan.The manipulation may be effected by activating at least one manipulationdevice operatively connected to an optical element of the projectionobjective to actively change the optical effect of that manipulatedoptical element and, as a result, also to actively change the imagingproperties of the entire projection objective.

Embodiments may be characterized by the fact that a field curvature ofthe projection objective is dynamically varied according to a given timeprofile during the scanning operation. A very efficient way to adapt theimaging properties of the projection objective to a time-dependentvariation of the substrate surface shape and/or substrate surfaceposition within the exposure area is thereby possible. Alternatively, orin addition, a dynamic variation of field curvature of the projectionobjective during scanning may also be employed to adapt the imagingproperties to a variation of the surface shape and/or surface positionof the pattern on the object side of the projection objective. Effectsof mask bending caused e.g. by the influence of gravity and/or by forcesand moments applied to a mask in a mask holder may thereby becompensated at least partly.

In systems subject to gradual drift of optical properties with time anoptical property, such as the focus position, may vary slowly (with asmall time constant) in one direction such that the optical propertyvaries little, and in only one direction, during a single scanningoperation. In contrast, in embodiments with highly dynamic compensationat least one aberration of the projection objective is changedsuccessively in two opposite directions between a beginning and an endof the scanning operation. In other words, the direction of change maychange one or more times during a single scanning operation. In doingso, negative effects of dome-like or valley-like surface unevenness withtypical size in the order of the size of an exposure area may becompensated for.

In general, it may be difficult to vary one imaging aberration in anisolated fashion without influencing other imaging aberrations.Therefore, other aberrations, specifically other field aberrations suchas distortion, coma, the field profile of defocus, astigmatism andsuperpositions thereof may be varied synchronously as the fieldcurvature is varied.

In some embodiments, the step of changing at least one imaging propertyof the projection objective includes changing in a spatially resolvingmanner the optical effect caused by at least one field element of theprojection objective. The term “field element” as used here relates toan optical element which includes at least one optical surface arrangedin a projection beam path optically close to a field surface of theprojection objective. Actively changing the optical effect caused by afield element allows to exert relatively large corrective effects onfield aberrations, such as a field curvature and distortion, while atthe same time the influence of the change on wavefront aberrations, suchas astigmatism, coma, spherical aberration and higher order aberrationsmay be kept small. Manipulating a field element may therefore be used tochange in a targeted manner field aberrations substantially withoutinducing parasitic pupil aberrations at the same time.

There are various ways to characterize a position “optically close to afield surface”. In general, it may be useful to define the axialposition of optical surface, such as surfaces of lenses or mirrors, bythe paraxial sub-aperture ratio SAR which is defined here as:SAR=(sign CRH)*(MRH/(|MRH|+|CRH|)).

In this definition, parameter MRH denotes the paraxial marginal rayheight and parameter CRH denotes the paraxial chief ray height of theimaging process. For the purpose of this application, the term “chiefray” (also known as principle ray) denotes a ray running from anoutermost field point (farthest away from the optical axis) of aneffectively used object field to the center of the entrance pupil. Inrotational symmetric systems the chief ray may be chosen from anequivalent field point in the meridional plane. In projection objectivesbeing essentially telecentric on the object side, the chief ray emanatesfrom the object surface parallel or at a very small angle with respectto the optical axis. The imaging process is further characterized by thetrajectory of marginal rays. A “marginal ray” as used herein is a rayrunning from an axial object field point (field point on the opticalaxis) to the edge of an aperture stop. That marginal ray may notcontribute to image formation due to vignetting when an off-axiseffective objective field is used. Both chief ray and marginal ray areused here in the paraxial approximation. The radial distances betweensuch selected rays and the optical axis at a given axial position aredenoted as “chief ray height” (CRH) and “marginal ray height” (MRH),respectively. A ray height ratio RHR=CRH/MRH may be used to characterizeproximity to or distance from field surfaces or pupil surfaces.

A definition of the paraxial marginal ray and the paraxial chief ray maybe found, for example, in: “Fundamental Optical Design” by Michael J.Kidger, SPIE PRESS, Bellingham, Wash., USA (Chapter 2), which documentis incorporated herein by reference.

The paraxial sub-aperture ratio as defined here is a signed quantityproviding a measure describing the relative proximity of a positionalong an optical path to a field plane or a pupil plane, respectively.In the definition given above, the paraxial sub-aperture ratio isnormalized to values between −1 and 1, where the condition SAR=0 holdsfor a field plane and a point of discontinuity with a jump from SAR=−1to SAR=+1 or from SAR=+1 to SAR=−1 corresponds to a pupil plane.Therefore, optical surfaces being positioned optically close to a fieldplane (such as the object surface or the image surface) arecharacterized by values of the paraxial sub-aperture ratio close to 0,whereas axial positions optically close to a pupil surface arecharacterized by absolute values for the paraxial sub-aperture ratioclose to 1. The sign of the paraxial sub-aperture ratio indicates theposition of the plane optically upstream or downstream of a plane. Forexample, the paraxial sub-aperture ratio a small distance upstream of apupil surface and a small distance downstream of a pupil surface mayhave the same absolute value, but opposite signs due to the fact thatthe chief ray height changes its sign upon transiting a pupil surface.

Bearing these definitions in mind, a field element may be defined as anoptical element having at least one optical surface arranged opticallynearer to a field surface than to a pupil surface. Positions opticallyclose to a field surface may be characterized by an absolute value ofthe ray height ratio RHR=CRH/MRH>1.

In other words, typical optical surfaces “optically close to a fieldsurface” are in positions where the absolute value of the chief rayheight CRH exceeds the absolute value of the marginal ray height MRH.

In another description the surface “optically close to a field surface”may be characterized by values of the paraxial sub-aperture ratio SARclose to zero. For example, the paraxial sub-aperture SAR may be in therange between 0 and about 0.4 or between 0 and 0.2 at an optical surfaceof a field element.

In some embodiments, a field element is arranged immediately adjacent tothe next field surface such that there is no optical element arrangedbetween the field element and the closest field surface.

The field element may be an optical element of the projection objective.It is also possible that a field element is arranged between the objectsurface of the projection objective and the object-side entry surface ofthe projection objective or between the image-side exit surface of theprojection objective and the image surface.

The field element provided for aberration manipulation may be atransparent optical element in the projection beam path. In this case,the optical effect of the transparent optical element may be varied orchanged in a spatially resolving manner (depending on the locationwithin the optically used area) by changing a spatial distribution ofrefractive power in the optically used area. For this purpose thetwo-dimensional distribution of refractive index of a transparentmaterial of the transparent optical element may be changed in a targetedmanner. This can be caused, for example, by a targeted local heating ofthe transparent material. Potentially suitable constructions aredisclosed, for example, in WO 2008/034636 A2, the disclosure of which isincorporated herein by reference. Alternatively, or in addition, thelocal distribution of refractive power may be changed by changing aspatial distribution of local surface curvature of an optical surface ofthe transparent optical element. This can be caused by targeteddeformation of the optical element, for example. Examples ofmanipulators are shown e.g. in US 2003/0234918 A1. A combination ofchanging the refractive index and changing the local distribution ofsurface curvature may be used. Further, manipulation may be effected byrelative displacement of aspheric surfaces having complementary shape,such as shown, e.g. in EP 0 851 304 B1. Electro-optical manipulators mayalso be utilized. The construction and operation of conventionalmanipulators may have to be modified to allow sufficient dynamics.

Although the transparent optical field element may be designed as a lenshaving a substantial refractive power in each of its configurations, thetransparent optical field element may also be shaped as a plane parallelplate having substantially no overall optical power. Such plate-likefield element may be arranged very close to a field surface in manytypes of projection objectives, for example close to the object surfaceor—in projection objectives forming at least one real intermediateimage—close to an intermediate image,

In some embodiments, at least one field element is a mirror having areflective surface arranged in the projection beam path optically closeto a field surface. Changing the optical properties of the mirror mayinclude changing a surface profile of the reflective surface in anoptically used area. A mirror manipulator may be operatively connectedto the field mirror and may be configured in such a way allowing to varythe shape of the reflective surface of the field mirror in one or twodimensions.

The number of degrees of freedom for dynamically changing imagingproperties of the projection objective may be increased by providing atleast two field elements, which may be manipulated in a prescribedcoordinated manner independently of each other. For example, theprojection objective may include two reflective field elements (fieldmirrors), each optically close to the field surface.

Each of the field mirrors may be assigned a mirror manipulatorconfigured to vary the shape of the reflective surface of the fieldmirror in a target fashion.

U.S. Pat. No. 7,385,756 by the applicant discloses catadioptric in-lineprojection objectives having two intermediate images and two concavemirrors, each arranged near an intermediate, i.e. relatively close to afield surface. Both concave mirrors may be employed as manipulators. Thedisclosure of this document is incorporated herein by reference.

At least one field element may be arranged close to the object surface.In some embodiments, a transparent field element forms a first elementof the projection objective immediately following the object surfacesuch that an entry surface of the field element forms the entry surfaceof the projection objective. A strong influence on field aberrations maythereby be obtained substantially without influencing pupil aberrationsto a significant degree. In projection objectives creating at least onereal intermediate image between object surface and intermediate imagesurface a field element may be arranged optically close to theintermediate image. In some embodiments, the projection objective has atleast two or exactly two intermediate images. A field element may bearranged optically close to each of the intermediate image surfaces,which are field surfaces of the projection objective.

Regarding the dynamics of typical manipulations it may be consideredthat modern scanning systems may operate with scanning speeds betweenabout 0.2 m/s and about 2 m/s, for example. The scanning speed may be inthe order of 700 mm/s to 800 mm/s Typical scanning path lengths may bein the order of several 10 mm (e.g. 30 mm-40 mm) This allows forexposure times in the order of some 10 ms (milliseconds) per exposurearea (die). Considering a typical value of about 50 ms scanning time perdie the dynamics of manipulation may be in the order of 20 Hz or more intypical cases, such as 40 Hz or more, or 60 Hz or more, or 80 Hz ormore, or 100 Hz or more, or 120 Hz or more.

Regarding the amplitude of manipulations it is considered that a localchange in field curvature peak-to-valley in the order of ±45 nm may besufficient in many high NA systems to achieve a reasonable degree ofcompensation. Such high NA systems may have maximum usable NA in theorder of NA=0.8 or higher (e.g. NA≧0.9, NA≧1, NA≧1.2, NA≧1.35).

In relative terms, it is presently considered that a rate of change inthe order of 10% of the depth of focus (DOF) within 10 ms (milliseconds)may be sufficient in many cases to achieve sufficient compensation. Insome embodiments, the field curvature is changed with a rate of changebetween about 0.5% and about 50% of the depth of focus (DOF) of theprojection objective within a 1 ms time interval

In embodiments configured to effectively compensate for negative effectsof substrate unevenness the method may further include: generatingsubstrate surface data representing a surface profile of the substratein the exposure area; generating manipulator control signals based onthe substrate surface data; and driving at least one manipulation deviceof the projection objective in response to the manipulator controlsignals to dynamically adapt the imaging properties of the projectionobjective to reduce imaging aberrations caused by the surface profile inthe exposure area.

The substrate surface data may be generated by measuring a topography ofthe substrate surface in a measuring area including the exposure area.Alternatively, the substrate surface data may be derived from datacontained in a look-up-table representing a topography of the substratesurface in a measuring area including the exposure area. Data of thelook-up-table may be gathered in advance experimentally or from abinitio calculations. For example, a correction of field curvature may beperformed based on a preceding measurement of substrate surfacetopography, while compensation concepts may be based on data containedin a look-up table.

Imaging aberrations induced by the unevenness of a wafer surface, forexample, may thereby be compensated for in a very efficient way.Measurement may be performed by a conventional method. Wafer unevennessmay be introduced, for example, by a substrate transfer process, asdescribed for example in: H. W. van Zeijl, J. Su, J. Slabbekoorn, F. G.C. Bijnen, “Lithographic Alignment Offset Compensation for SubstrateTransfer Process”, Proc. STW/SAFE, Veldhoven, The Netherlands, 2005, pp121-126.

In embodiments configured to effectively compensate for negative effectsof unevenness of the mask, e.g. caused by gravity induced reticlebending, the method may further include: generating mask surface datarepresenting a surface profile of the mask in an mask area correspondingto the exposure area; generating manipulator control signals based onthe mask surface data; and driving at least one manipulation device ofthe projection objective in response to the manipulator control signalsto dynamically adapt the imaging properties of the projection objectiveto reduce imaging aberrations caused by the surface profile in the maskarea.

The mask surface data may be generated by measuring a topography of themask surface in a measuring area including the mask area correspondingto the exposure area. Alternatively, the mask surface data may bederived from data contained in a look-up-table representing a topographyof the mask surface in a measuring area including the mask areacorresponding to the exposure area. Data of the look-up-table may begathered in advance, e.g. experimentally or from ab initio calculations.

Imaging aberrations induced by the unevenness of a mask surface, forexample, may thereby be compensated for in a very efficient way.

The previous and other properties can be seen not only in the claims butalso in the description and the drawings, wherein individualcharacteristics may be used either alone or in sub-combinations as anexemplary embodiment of the disclosure and in other areas and mayindividually represent advantageous and patentable embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic drawing of an exemplary embodiment of aprojection exposure apparatus for microlithography having anillumination system and a projection objective;

FIGS. 2A-2B shows a schematic illustrating the effect of differentamounts of field curvature on the imaging process;

FIGS. 3A-3C shows schematically the conventional principle of correctingdefocus errors using a wafer stage capable of lifting or lowering asubstrate surface parallel to the optical axis;

FIG. 4 shows in 4A an axial view of a semiconductor wafer with aplurality of exposure areas (dies), in 4B one selected rectangularexposure area, and in 4C a vertical section through the exposure areashowing an uneven substrate surface;

FIG. 5 shows a Zernike spectrum representing the effect of anessentially quadratic surface profile in x direction within an exposurearea on selected aberrations;

FIG. 6 shows in 6A the positions of a scanning field in an exposure areaat two different instances of time and in 6B the respective curvature ofthe image field of a projection objective dynamically manipulated toconform to the surface shape of the exposure area within the respectiveimage field position;

FIG. 7A-7B shows two axial sections of a first exemplary embodiment of acatadioptric projection objective including two concave mirrorsoptically close to intermediate images and associated with dynamicmirror manipulators;

FIG. 8 shows a diagram indicating aberrations induced by an unevensubstrate surface and decomposed into contributions described byZernicke coefficients for three different compensation scenarios;

FIG. 9 shows schematically surface deformations of the two concavemirrors shown in FIG. 7 which compensate partly aberrations induced byan uneven substrate surface topography;

FIG. 10A-10B shows sections of a second exemplary embodiment of acatadioptric projection objective including a transparent manipulatorelement immediately adjacent to the object surface;

FIG. 11 shows a diagram indicating aberrations induced by an unevensubstrate surface and decomposed into contributions described byZernicke coefficients for three different compensation scenarios;

FIG. 12 shows a schematic three-dimensional diagram showing asaddle-shaped deformation of a reticle;

FIG. 13 shows diagram indicating aberrations induced by an unevenreticle surface and decomposed into contributions described by Zernikecoefficients for two different compensation scenarios in an exemplaryembodiment having two deformable field mirrors;

FIG. 14 shows and comparative view of three correction scenarios for ananamorphotic reticle bending in an exemplary embodiment having atransparent manipulator element close to the object surface, and

FIG. 15 shows a detail of an exemplary embodiment including two foldingmirrors which can be deformed independently from each other by mirrormanipulators.

DETAILED DESCRIPTION

In the following description, the term “optical axis” refers to astraight line or a sequence of a straight-line segments passing throughthe centers of curvature of optical elements. The optical axis can befolded by folding mirrors (deflecting mirrors) such that angles areincluded between subsequent straight-line segments of the optical axis.In the examples presented below, the object is a mask (reticle) bearingthe pattern of a layer of an integrated circuit or some other pattern,for example, a grating pattern. The image of the object is projectedonto a wafer serving as a substrate that is coated with a layer ofphotoresist, although other types of substrates, such as components ofliquid-crystal displays or substrates for optical gratings, are alsofeasible.

Where tables are provided to disclose the specification of a designshown in a figure, the table or tables are designated by the samenumbers as the respective figures. Corresponding features in the figuresare designated with like or identical reference identifications tofacilitate understanding. Where lenses are designated, an identificationL3-2 denotes the second lens in the third objective part (when viewed inthe radiation propagation direction).

FIG. 1 shows schematically a microlithographic projection exposuresystem in the form of a wafer scanner WS, which is provided forfabricating large scale integrated semiconductor components viaimmersion lithography in a step-and-scan mode. The projection exposuresystem includes as primary radiation source S an Excimer laser having anoperating wavelength of 193 nm. Other primary radiation sources are usedin other exemplary embodiments, for example emitting at about 248 nm,157 nm or 126 nm Radiation sources for the extreme ultraviolet (EUV)spectral range may also be utilized in connection with purely reflective(catoptric) optical systems. An illumination system ILL opticallydownstream of the light source generates, in its exit surface ES, alarge, sharply delimited, homogeneously illuminated illumination fieldIF that is adapted to the desired telecentric properties of thedownstream projection objective PO. The illumination system ILL hasdevices for selection of the illumination mode and, in the example, canbe changed over between conventional on-axis illumination with avariable degree of coherence, and off-axis illumination, particularlyannular illumination (having a ring shaped illuminated area in a pupilsurface of the illumination system) and dipole or quadrupoleillumination.

Arranged downstream of the illumination system is a device RS forholding and manipulating a mask M in such a way that a pattern formed onthe mask lies in the exit surface ES of the illumination system, whichcoincides with the object surface OS of the projection objective PO. Thedevice RS—usually referred to as “reticle stage”—for holding andmanipulating the mask contains a mask holder and a scanner driveenabling the mask to be moved parallel to the object surface OS of theprojection objective or perpendicular to the optical axis of projectionobjective and illumination system in a scanning direction (y-direction)during a scanning operation.

The reduction projection objective PO is designed to image an image of apattern provided by the mask with a reduced scale of 4:1 onto a wafer Wcoated with a photoresist layer (magnification |β|=0.25). Otherreduction scales, e.g. 5:1 or 8:1 are possible. The wafer W serving as alight-sensitive substrate is arranged in such a way that themacroscopically planar substrate surface SS with the photoresist layeressentially coincides with the planar image surface IS of the projectionobjective. The wafer is held by a device WST (wafer stage) including ascanner drive in order to move the wafer synchronously with the mask Min parallel with the latter. The wafer stage includes a z-manipulatormechanism to lift or lower the substrate parallel to the optical axis OAand a tilting manipulator mechanism to tilt the substrate about two axesperpendicular to the optical axis.

The device WST provided for holding the wafer W (wafer stage) isconstructed for use in immersion lithography. It includes a receptacledevice RD, which can be moved by a scanner drive and the bottom of whichhas a flat recess for receiving the wafer W. A peripheral edge forms aflat, upwardly open, liquid tight receptacle for a liquid immersionmedium IM, which can be introduced into the receptacle and dischargedfrom the latter by devices that are not shown. The height of the edge isdimensioned in such a way that the immersion medium that has been filledin can completely cover the surface SS of the wafer W and the exit-sideend region of the projection objective PO can dip into the immersionliquid given a correctly set operating distance between objective exitand wafer surface.

The projection objective PO has a planoconvex lens as the last opticalelement nearest to the image surface IS. The planar exit surface of theplanoconvex lens is the last optical surface of the projection objectivePO. During operation of the projection exposure system, the exit surfaceof the last optical element is completely immersed in the immersionliquid IM and is wetted by the latter.

In other exemplary embodiments the exit surface is arranged at a workingdistance of a few millimeters above the substrate surface SS of thewafer in such a way that there is a gas-filled gap situated between theexit surface of the projection objective and the substrate surface (drysystem).

As shown schematically in the inset figure of FIG. 1, the illuminationsystem ILL is capable of generating an illumination field having arectangular shape. The size and shape of the illumination fielddetermines the size and shape of the effective object field OF of theprojection objective actually used for projecting an image of a patternon a mask in the image surface of the projection objective. Theeffective object field has a length A* parallel to the scanningdirection and a width B*>A* in a cross-scan direction perpendicular tothe scanning direction and does not include the optical axis (off-axisfield).

The projection objective PO may include a plurality of schematicallyindicated lenses (typical numbers of lenses are often more than 10 ormore than 15 lenses) and, if appropriate, other transparent opticalcomponents. The projection objective may be purely dioptric (lensesonly). The projection objective may include at least one powered mirror,such as at least one concave mirror, in addition to lenses (catadioptricprojection objective).

For many applications in the field of microlithograpy the image-sidenumerical aperture of the projection objective is NA>0.6, and in manyexemplary embodiments NA is between approximately NA=0.65 and NA=0.95,which can be achieved by dry objectives. Using an immersion systemallows obtaining NA values NA≧1, such as NA≧1.1 or NA≧1.2 or NA≧1.3 orNA≧1.4 or NA≧1.5 or NA≧1.6 NA≧1.7 or above. Typical resolutions down toabout 150 nm, or 130 nm, or 100 nm, or 90 nm or less are also possiblebasically depending on the combination of image-side NA and thewavelength of the radiation source.

The projection objective PO is an optical imaging system designed toform an image of an object arranged in the object surface OS in theimage surface, which is optically conjugate to the object surface. Theimaging may be obtained without forming an intermediate image, or viaone or more intermediate images, for example two intermediate images.

Every optical system has associated with it a sort of basic fieldcurvature, which is conventionally called the Petzval curvature, whichis now explained in more detail with reference to FIG. 2A. When there isno astigmatism, the sagittal and tangetial image surfaces coincide witheach other and lay on the Petzval surface. Positive lenses (lenses withpositive refractive power) introduce inward curvature of the Petzvalsurface to a system, and negative lenses (lenses with negativerefractive power) introduce backward curvature. The Petzval curvature,1/R_(P), is given by the Petzval sum 1/R_(P), which is the reciprocal ofthe Petzval radius R_(P), which is the radius of curvature of thePetzval surface.

For example, he condition R_(P)<0 holds for a positive lens,corresponding to an inward curvature of the Petzval surface. Therefore,the image of a planar object will be concave towards the radiationdirection, which condition is typically referred to as “undercorrection”of field curvature. In contrast, the condition R_(P)>0 holds for aconcave mirror, corresponding to overcorrection of the field curvature.A flat image of a flat object is obtained by an imaging system withR_(P)=0. These conditions are shown schematically for projectionobjective PO in FIG. 2A.

The Petzval surface and the paraxial image surface coincide on theoptical axis. The curvature of the Petzval surface may result in asituation where the Petzval surface departs from the ideal image surfacefor field points further away from the optical axis. The fact that thePetzval surface is curved transforms to a longitudinal departure p ofthe Petzval surface from the ideal image surface (which is usually flat)at a field point at the outer edge of the image field (at maximum imagefield height y′), measured parallel to the optical axis in the imagespace. The term “image field curvature” is conventionally used to referto such longitudinal departure (or sag) at maximum image field heighty′, and may not be confused with the “curvature of the image field”,which is the reciprocal of the radius of curvature of the image field.

Note that FIG. 2 is purely schematic and not to scale for any of thefeatures discussed in connection with the figure.

Conditions on the image-side of the objective are now explained inconnection with FIG. 2B. For the purpose of this application the imagefield size may be characterized by a maximum image field height y′,which corresponds to the radius of the (circular) “design image field”of the objective. The design image field IF_(D) includes all fieldpoints of the image surface for which the imaging fidelity of theobjective is sufficiently good for the intended lithographic process.With other words: all imaging aberrations are corrected sufficiently forthe intended application within zones having radial coordinates equal toor smaller than the maximum image field height y′, whereas one or moreaberrations may be higher than a desired threshold value for fieldpoints outside the design image field IF_(D).

In general, not all field points within the design image field IF_(D)are used for a lithographic process. Instead, exposure is only performedusing field points lying within an effective image field IF, whichshould be sufficiently large in size to allow reasonably sizedsubstrates to be exposed in a lithographic process. The effective imagefield desirably fits into the design image field IF_(D) in order toinclude only field points for which the objective is sufficientlycorrected and for which no vignetting occurs. There are various ways tofit an effective image field into a design image field depending on thedesign of an objective.

In projection exposure systems designed for a scanning operation, aslit-shaped effective image field IF is used. FIG. 2B shows an exampleof a rectangular effective off-axis image field IF, which may beutilized in connection with exemplary embodiments of obscuration-freecatadioptric projection objectives as discussed exemplarily below. Insoother embodiments, an effective image field with an arcuate shape(sometimes noted as annular field or ring field) may be used. The sizeof an effective image field may generally be described in terms of alength A parallel to the scanning direction and a width B>Aperpendicular to the scanning direction, thereby defining an aspectratio AR=B/A>1. In many exemplary embodiments the aspect ratio may be inthe range from 2:1 to 10:1, for example.

State-of-the-art scanning projection exposure systems may be equippedwith a wafer stage allowing to move the substrate table parallel to theoptical axis (z-manipulation) and also to tilt the wafer table about twomutually perpendicular axes perpendicular to the optical axis (x-tiltand y-tilt). The system may be operated to shift and/or tilt the wafertable between exposure steps in order to adjust the position of thesubstrate surface with respect to the focal region of the projectionobjective based on previous measurements of the substrate surface (seee.g. U.S. Pat. No. 6,674,510 B1).

However, those systems are not fully capable of accounting for localunevenness of the substrate surface within an exposure region. Instead,a compromise position of the substrate surface with respect to the focalregion of the projection objective will typically be obtained. Theinventors have recognized, however, that an uneven surface profilewithin an exposure area may cause deterioration of the quality of theimaging process, thereby leading to increased reject rates in themanufacturing of microstuctured semiconductor devices, for example.

FIGS. 3A-3C show schematically the conventional principle of acorrection using a wafer stage capable of lifting or lowering thesubstrate surface parallel to the optical axis (z manipulation). In FIG.3A the uneven substrate surface SS lies entirely outside the focusposition of the projection objective at z=0. Therefore, as seen in FIG.3B, the location of the focus FOC lies above the substrate surface ofthe wafer W. If the wafer stage is lifted in axial direction by apredefined amount (z-MAN), the uneven substrate surface may be broughtinto the region of best focus such that at least parts of the exposurearea are in focus, and defocus aberrations are reduced (3C). However,depending of the depth of focus it may occur that parts of the exposurearea are still out of focus beyond an acceptable threshold value,whereby contrast may deteriorate significantly at least in parts of theexposure area.

In the following the unevenness of the substrate surface will be treatedas a disturbance for the lithographic imaging process. For illustrationpurposes, FIG. 4A shows an axial view of a semiconductor wafer W havinga substrate surface SS subdivided in two large number of rectangularexposure areas EA (or dies) arranged in a network of adjacent dies whichare successively irradiated via the reticle, one at a time. Eachexposure area has a width EAX, corresponding to the width of theeffective image field of the projection objective in the cross-scandirection (x-direction) and a length EAY which may be equal, larger orsmaller than the width EAX and which is substantially greater than theheight A of the effective image field. FIG. 4B shows a vertical section(x-z section) through the substrate showing the multiply curvedsubstrate surface SS. It is contemplated that after global adjustment ofthe axial position and the tilt angle of the entire substrate the localunevenness of the substrate surface results in a surface profile havingan essentially parabolic (quadratic) field profile in x-direction atleast in a first approximation. The amount of deviation from a planarreference surface may be quantified by the peak-to-valley value pv inthe respective area, which is defined here as the difference betweenminimum and maximum profile height in the exposure area. It is furthercontemplated that the curvature of the substrate surface SS in theorthogonal y-direction (scanning direction) can be neglected due to thehigh aspect ratio of the slit-shaped effective image field (for example4<AR<6). This uneven surface profile of the substrate will thereforeinduce at least in a first approximation, a quadratic field profile ofthe defocus and, at the same time, a quadratic field profile of aspherical aberration due to the high image-side numerical aperture ofthe projection objective. Note that these aberrations are independent ofthe optical design of the projection objective, and are only dependentfrom the image-side NA of the projection objective.

In the following, the wavefront aberrations caused by the projectionsystem and/or induced by external conditions are expressed as a linearcombination of polynomials. In the optics field, several types ofpolynomials are available for describing aberrations, for example Seidelpolynomials or Zernike polynomials. Zernike polynomials are employed inthe following to characterize aberrations.

The technique of employing Zernike terms to describe wavefrontaberrations originating from optical surfaces deviating from beingperfectly spherical is a state-of-the art technique. It is also wellestablished that the different Zernike terms signify differentaberration phenomena including defocus, astigmatism, coma and sphericalaberration up to higher aberrations. An aberration may be expressed as alinear combination of a selected number of Zernike polynomials. Zernikepolynomials are a set of complete orthogonal polynomials defined on aunit circle. Polar coordinates are used, for example with ρ being thenormalized radius and θ being the azimuth angle. A wavefront aberrationW(ρ, θ) may be expanded in Zernike polynomials as a sum of products ofZernike terms and respective weighting factors (see e.g. Handbook ofOptical Systems: Vol. 2, Physical Image Formation, ed. By H Gross,Wiley-VCH Verlag GmbH & Co. KGaA, Chapter 20.2, (2005)). In a Zernikerepresentation, the Zernike polynomials Z1, Z2, Z3 etc. have certainmeanings identifying respective contributions to an overall aberration.For example, Z1=1 corresponds to a constant term (or piston term), Z2=ρcos θ corresponds to a distortion in x direction (or a wavefront tilt inx direction), Z3=ρ sin θ corresponds to distortion in y direction (or awavefront tilt in y direction), Z4=2ρ²−1 corresponds to a defocus(parabolic part), Z5=ρ² cos 2 θ 0 corresponds to astigmatism thirdorder, etc.

Zernike polynomials may also be used to characterize deviations of anoptical surface, such as a lens surface or a mirror surface, from anominal surface, e.g. a spherical surface.

FIG. 5 shows a Zernike spectrum representing the effect of anessentially quadratic surface profile in x-direction within an exposurearea for a substrate surface unevenness characterized by apeak-to-valley-value PV=100 nm in the exposure area. It is evident thatthe unevenness basically influences the defocus (Zernike coefficient Z4)and the spherical aberration (primary spherical aberration Z9, secondaryspherical aberration Z16 etc.), while at the same time the level ofother aberrations induced by the uneven (non-planar) surface profile iscomparatively small.

The following example shows how imaging aberrations induced by unevensubstrate surface within an exposure area of the scanning lithographyapparatus may be significantly reduced. Schematic FIG. 6A shows arectangular exposure area EA on an uneven wafer surface during ascanning operation at two different instant of time. At the firstinstant of time t₁ the illuminated slit-shaped effective image field IFof the projection objective is positioned close to the lower edge of theexposure area. As the scanning proceeds the wafer moves relative to thestationary projection objective in scanning direction (y-direction) suchthat the illuminated effective image field is at different positionspaced apart from the first position at a later instant of time, t₂.With typical values for scanning speeds (e.g. between about 0.2 m/s and2 m/s) and exposure area sizes with typical edge length in the order ofone or more centimeters, e.g. between 20 mm and 40 mm, the time intervalΔt involved to cover the entire exposure area with the illuminated imagefield in one scanning operation may typically range between about 10 msand 200 ms, for example.

FIG. 6B shows respective sections of the substrate in the x-z-planeperpendicular to the scanning direction. In each case, the image-sideend of the projection objective PO is shown, the exit-side of theprojection objective spaced apart by working distance from the unevensubstrate surface SS of the substrate. While the substrate surface has aconvex surface shape at t₁, the substrate surface is concavely curved ina spaced apart region traversed by the projection beam at a later timet₂>t₁.

The dashed line in each case represents the Petzval surface PS of theprojection objective, which is adapted to conform to the surfacetopography of the substrate within the effective image field to allowreducing the imaging aberrations caused by the uneven substrate surface.In the example, the projection objective is slightly undercorrected withrespect to field curvature at the moment t₁ and the correction status isdynamically changed to a slightly overcorrected condition at moment t₂.This significant variation of image field curvature of the imagingsystem is effective dynamically within fractions of a second adapted tothe scanning speed. The variation is effective based on surfacetopography measurements performed prior to the scanning operation, asexplained in more detail below.

Considering a typical value of about 50 ms scanning time per die thedynamics of manipulation may be in the order of 20 Hz or more, forexample.

In relative terms, it is presently considered that a rate of change inthe order of 10% of the depth of focus (DOF) within 10 ms (milliseconds)may be sufficient in many cases to achieve sufficient compensation. Insome exemplary embodiments, the defocus is changed with a rate of changebetween about 0.5% and about 50% of the depth of focus (DOF) of theprojection objective within a 1 ms time interval.

The rate of change of imaging aberrations, such as field curvatureand/or distortion, brought about by active manipulation of one ore moreoptical elements of the projection objective may be orders of magnitudefaster than time-dependent changes which may occur during the use of theprojection system caused by environmental changes, such as changes ofenvironmental pressure and/or temperature, and/or changes induced byheating of the system.

The effects of targeted manipulation of one or more optical elements ofprojection objectives situated relatively close to a respective fieldsurface (field elements) will now be described in more detail usingworking examples of catadioptric projection objectives adapted toimmersion lithography at an NA>1. The respective exposure systems may beequipped with state-of-the-art manipulators for moving the mask and/orthe reticle parallel to the optical axis and for tilting the mask and/orthe wafer about one or more tilt axis perpendicular to the optical axis.In addition, the exposure systems may be equipped with wavelengthmanipulators to shift the center wavelength λ of the system within amanipulation range Δλ adapted to the status of chromatic correction ofthe respective projection objectives.

In each case, one or more additional manipulators are specificiallyprovided to account for effects of substrate surface unevenness and/orunevenness of the reticle, as explained more detailed below.

FIGS. 7A and 7B show two axial sections of a first exemplary embodimentof a catadioptric projection objective 700. Given a demagnifying imagingscale of 4:1 (β=−0.25) the projection objective is telecentric on theobject side and on the image side has an image-side numerical apertureNA=1.35. The effective image field size is 26 mm by 5.5 mm. Thespecification is given in tables 7, 7A. FIG. 7A shows a section in thex-z-plane, while FIG. 7B shows the respective section in the y-z-planeperpendicular thereto. The specification data for the system are takenfrom the exemplary embodiment shown in FIG. 3 of US 2008/0174858 A1. Thecorresponding disclosure is incorporated herein by reference.

Projection objective 700 is designed to project an image of a pattern ona reticle arranged in the planar object surface OS (object plane) intothe planar image surface IS (image plane) on a reduced scale, forexample, 4:1, while creating exactly two real intermediate images IMI1,IMI2. The rectangular effective object field OF and image field IF areoff-axis, i.e. entirely outside the optical axis OA. A first refractiveobjective part OP1 is designed for imaging the pattern provided in theobject surface into the first intermediate image IMI1. A second,catoptric (purely reflective) objective part OP2 images the firstintermediate image IMI1 into the second intermediate image IMI2 at amagnification close to 1:(−1). A third, refractive objective part OP3images the second intermediate image IMI2 onto the image surface IS witha strong reduction ratio.

Projection objective 700 is an example of a “concatenated” projectionobjective having a plurality of cascaded objective parts which are eachconfigured as imaging systems and are linked via intermediate images,the image (intermediate image) generated by a preceding imaging systemin the radiation path serving as object for the succeeding imagingsystem in the radiation path. The succeeding imaging system can generatea further intermediate image (as in the case of the second objectivepart OP2) or forms the last imaging system of the projection objective,which generates the “final” image field in the image plane of theprojection objective (like the third objective part OP3).

The path of the chief ray CR of an outer field point of the off-axisobject field OF is indicated in FIG. 7B in order to facilitate followingthe beam path of the projection beam.

Three mutually conjugated pupil surfaces P1, P2 and P3 are formed atpositions where the chief ray CR intersects the optical axis. A firstpupil surface P1 is formed in the first objective part between objectsurface and first intermediate image, a second pupil surface P2 isformed in the second objective part between first and secondintermediate image, and a third pupil surface P3 is formed in the thirdobjective part between second intermediate image and the image surfaceIS.

The second objective part OP2 includes a first concave mirror CM1 havingthe concave mirror surface facing the object side, and a second concavemirror CM2 having the concave mirror surface facing the image side. Theused parts of the mirror surfaces, i.e. those areas of the mirrorsurfaces which are illuminated during operation, are both continuous orunbroken, i.e. the mirrors do not have a hole or bore in the illuminatedregion. The mirror surfaces facing each other define a catadioptriccavity, which is also denoted intermirror space, enclosed by the curvedsurfaces defined by the concave mirrors. The intermediate images IMI1,IMI2 are both situated inside the catadioptric cavity well apart fromthe mirror surfaces.

The objective 700 is rotational symmetric and has one straight opticalaxis OA common to all refractive and reflective optical components(“In-line system”). There are no folding mirrors. An even number ofreflections occurs. Object surface and image surface are parallel. Theconcave mirrors have small diameters allowing to arrange them closetogether and rather close to the intermediate images lying in between.The concave mirrors are both constructed and illuminated as off-axissections of axial symmetric surfaces. The light beam passes by the edgesof the concave mirrors facing the optical axis without vignetting. Bothconcave mirrors are positioned optically remote from a pupil surfacerather close to the next intermediate image. The objective has anunobscured circular pupil centered around the optical axis thus allowinguse as projection objectives for microlithography.

Both concave mirrors CM1, CM2 are arranged optically close to a fieldsurface formed by the next intermediate image, as indicated by thefollowing table A giving the data for marginal ray height MRH, chief rayheight CRH, ray height ratio RHR and sub-aperture ratio SAR:

TABLE A MRH CRH RHR SAR CM1 18.33 −147.93 −8.1 −0.11 CM2 13.68 119.00+8.7 +0.10

A position optically close to a field surface may also be characterizedby the shape of the projection beam at the respective surface. Where anoptical surface is optically close to a field surface, the crosssectional shape of the projection beam deviates significantly from thecircular shape typically found in a pupil surface or in proximitythereto. In this context, the term “projection beam” describes thebundle of all rays running from the effective object field in the objectsurface towards the effective image field in the image surface. Surfacepositions optically close to a field surface may be defined as positionswhere the beam diameters of the projection beam in two mutuallyperpendicular directions orthogonal to the propagation direction of thebeam may deviate by more than 50% or more than 100% or more from eachother. Typically, an illuminated area on an optical surface opticallyclose to a field surface will have a shape strongly deviating from acircle end resembling a high aspect ratio rectangle corresponding to adesirable field shape in scanning lithographic projection apparatus. Itcan be seen from FIG. 9 below that the illuminated areas on both thefirst and second concave mirror are essentially rectangular with roundededges, the rectangle having about the aspect ratio AR of the effectiveobject and image field. The rectangular shape of the effective objectfield OF may be seen from the comparison between FIGS. 7A and 7B,wherein in FIG. 7A the object field is cut along the long side(x-direction) whereas in FIG. 7B the object field is cut parallel to thescanning direction i.e. parallel to the short edge of the rectangulareffective object field.

A primary function of the two concave mirrors CM1 and CM2 is to correctthe Petzval sum by providing an overcorrecting contribution to Petzvalsum counteracting the undercorrecting influence of positive refractivepower of lenses. The contribution of the concave mirrors to fieldcurvature may be varied dynamically by varying the surface curvature ofthe reflective surfaces according to a defined spatial and time profile.For this purpose, each of the first and second concave mirrors isassociated with a mirror manipulator MM1, MM2, respectively, bothconfigured for a two-dimensional deformation (deformation with spatialresolution in two dimensions) of the associated concave mirror during ascanning operation. The mirror manipulators may be identical ordifferent in construction.

In the example, each of the concave mirrors has a highly reflective (HR)coating on a flexible portion of a mirror substrate. A number ofactuators (represented by arrows) of a mirror manipulator MM1 or MM2 areoperatively coupled to the back side of the flexible portion. Theactuators are controlled by a mirror control unit MCU, which may be anintegral part of central control unit of the projection exposureapparatus. The manipulator control unit is connected to receive signalsrepresenting a desired deformation of the mirror surface. The mirrormanipulator and corresponding control unit may be designed essentiallyas disclosed in applicants patent applications US 2002/0048096 A1 or US2005 0280910 A1 (corresponding to WO 03/98350 A2), for example. Thecorresponding disclosures are incorporated into this application byreference.

Any suitable construction of the pupil mirror manipulator may be usedinstead, for example manipulators using electromechanical actuators,such as piezoelectrical elements, actuators responding to fluid pressurechanges, electric and/or magnetic actuators. These actuators may be usedto deform a continuous (unbroken) mirror surface as described. Themirror manipulator may also include one or more heating elements and/orcooling elements effecting local temperature changes of the mirrorleading to a desired deformation of the mirror surface. Resistanceheaters or Peltier elements may be used for that purpose.

The effect of two-dimensional deformation of both the first and thesecond concave mirror CM1, CM2 during a scanning operation was simulatedfor a wafer substrate having an uneven substrate surface with a concaveparabolic profile in cross-scan direction (x-direction) with 45 nmdifference between maximum height and minimum height (45 nmpeak-to-valley), substantially as schematically shown in FIG. 4B. Threecorrection scenarios SC1, SC2 and SC3 have been simulated.

In a first scenario SC1 the substrate unevenness has been corrected byactive manipulation of the wafer z-position and wafer tilt status,basically as explained in connection with FIG. 3, in a conventionalmanner. In a second scenario SC2 a number of active manipulations onoptical elements including relative displacements of lenses parallel tothe optical axis and tilting of lenses have been performed in addition.

In a third scenario SC3 the surface shapes of both the first and thesecond concave mirror CM1, CM2 have been two-dimensionally deformed in atime-dependent manner during the scanning operation to reduceaberrations caused by the unevenness of the substrate surface. FIG. 8shows a comparative diagram of aberrations induced by the quadraticunevenness (PV=45 nm) in the mentioned three scenarios SC1, SC2, SC3.The overall induced aberrations are decomposed in contributionsdescribed by Zernike coefficients, which are shown as the values on theabscissa of FIG. 8. The ordinate shows the scanned aberrations SCA innm. A significant improvement of the scanned aberrations is immediatelydiscernible for the case where two mirrors, each optically close to afield surface (intermediate images), are dynamically deformed during thescanning operation to reduce aberrations. Both the defocus aberration(Z4) and the wavefront tilt aberration (Z2/3) could be reducedsignificantly both with respect to the standard scenario SC2 and evenmore reduced with respect to the first scenario SC1, where only anoptimization of wafer position with respect to axial position and tiltis used. Both defocus (Z4) and wavefront tilt (Z2/3) can be reduced byabout 90% relative to the standard scenario SC2, while even betterimprovement by about 95% is obtained relative to the pure wafer scenarioSC1. For example, defocus aberration Z4 could be reduced from about 12nm to about 0.6 nm by dynamic deformation of the two concave mirrors.Likewise, a significant improvement is obtained for astigmatism (Z5/6),which could be reduced by about 90% relative to the standand scenarioSC2.

These values indicate that the dynamic deformation of field elementsreduces significantly the contribution of substrate surface unevennessto the focus budget, where it is presently amongst the dominatingcontributions. A significant improvement of process latitude in thelithographic process is thereby obtained. At the same time, the desiredproperties related to focus errors can be relaxed with regard to othercontributing effects, such as lens heating or the like.

FIG. 9 shows schematically the correction deformations applied to thefirst concave mirror CM1 (FIG. 9A) and second concave mirror CM2 (FIG.9B) involved to obtain the improvement. In each case, the peak-to-valley(PV) deformations of the reflective surfaces are relatively small andmay range in a region below 200 nm, for example. In this specific case,the PV deformation is about 80 nm for CM1 and about 160 nm for CM2.Further, the figures exemplarily show that relatively long-wavedeformations are effective to reduce the aberrations caused by thelong-wave quadratic deformation of the substrate surface in thecross-scan direction. This indicates that the deviation of the concavebase profile of the mirrors need not be very complex in many cases suchthat the construction of respective mirror manipulators may berelatively simple. In general, a multi-Zernike-mirror manipulator may beused in each case, allowing targeted deformations of the reflectivesurface which may be decomposed in Zernike coefficients up to highvalues, such as between Z2 and Z49, for example. Instead, deformationswith less complex structure may be sufficient in many cases, allowinguse of less complex mirror manipulators.

The compensation mechanism can be implemented in a projection exposureapparatus including a projection objective which has one or moremanipulators associated with one or more optical elements of theprojection objective by integrating the manipulator(s) into a controlsystem configured to change imaging properties of the projectionobjective actively during the scanning operation according to a givenprofile to change dynamically at least aberration of the projectionobjective between a beginning and an end of the scanning operation. Themanipulator associates with an optical element may be connected to acontrol unit generating manipulator control signals which initiaterespective changes of the optical effect of the manipulated opticalelement. The manipulator control signals may be generated in differentways. In some exemplary embodiments, a measuring system is providedwhich allows measuring the surface topography of a substrate surface ina measuring area including the exposure area. Alternatively, thesubstrate surface data may also be derived from data contained in alook-up-table representing a measured or calculated topography of thesubstrate surface in an area including the exposure area.

Where the system is configured to allow compensation of distortion ofother aberrations caused by non-ideal surface shape of the mask,corresponding measures can be implemented to account for non-evensurface shape of the reticle. Corresponding mask surface datarepresenting a surface profile of the mask in a mask area correspondingto the exposure area may be generated either based on a measurement orbased on data from a look-up-table, for example. Mask surface data maybe processed by the control unit to generate manipulator control signalswhich are then used to control at least one manipulation device withinthe projection objective to dynamically adapt the imaging properties ofthe projection objective in a compensating way to reduce imagingaberrations caused by the surface profile in the mask area.

FIGS. 10A and 10B show a second exemplary embodiment of a catadioptricprojection objective 1000 for immersion lithography at about X=193 nmaccording to a different design. Given a demagnifying imaging scale of4:1 (13=−0.25) the projection objective is telecentric on the objectside and on the image side has an image-side numerical aperture NA=1.32.The effective image field size is 26 mm by 5.5 mm. The specification isgiven in tables 10, 10A. FIG. 10A shows a section in the x-z-plane,while FIG. 10B shows the respective section in the y-z-planeperpendicular thereto. The specification data for the system are takenfrom the exemplary embodiment shown in FIG. 7 of US 2008/0174858 A1. Thecorresponding disclosure is incorporated herein by reference.

Projection objective 1000 is designed to project an image of a patternon a reticle arranged in the planar object surface OS (object plane)into the planar image surface IS (image plane) on a reduced scale, forexample, 4:1, while creating exactly two real intermediate images IMI1,IMI2. The rectangular effective object field OF and image field IF areoff-axis, i.e. entirely outside the optical axis OA. A first refractiveobjective part OP1 is designed for imaging the pattern provided in theobject surface into the first intermediate image IMI1. A second,catadioptric (refractive/reflective) objective part OP2 images the firstintermediate image IMI1 into the second intermediate image IMI2 at amagnification close to 1:(−1). A third, refractive objective part OP3images the second intermediate image IMI2 onto the image surface IS witha strong reduction ratio.

Projection objective 1000 is another example of a “concatenated”projection objective having a plurality of cascaded objective partswhich are each configured as imaging systems and are linked viaintermediate images, the image (intermediate image) generated by apreceding imaging system in the radiation path serving as object for thesucceeding imaging system in the radiation path. The sequence isrefractive-catadioptric-refractive (R-C-R).

The path of the chief ray CR of an outer field point of the off-axisobject field OF is drawn bold in FIG. 10B in order to facilitatefollowing the beam path of the projection beam.

Three mutually conjugated pupil surfaces P1, P2 and P3 are formed atpositions where the chief ray CR intersects the optical axis. A firstpupil surface P1 is formed in the first objective part between objectsurface and first intermediate image, a secand pupil surface P2 isformed in the second objective part between first and secondintermediate image, and a third pupil surface P3 is formed in the thirdobjective part between second intermediate image and the image surfaceIS.

The second objective part OP2 includes a single concave mirror CMsituated at the second pupil surface P2. A first planar folding mirrorFM1 is arranged optically close to the first intermediate image IMI1 atan angle of 45° to the optical axis OA such that it reflects theradiation coming from the object surface in the direction of the concavemirror CM. A second folding mirror FM2, having a planar mirror surfacealigned at right angles to the planar mirror surface of the firstfolding mirror, reflects the radiation coming from the concave mirror CMin the direction of the image surface, which is parallel to the objectsurface. The folding mirrors FM1, FM2 are each located in the opticalvicinity of, but at a small distance from the closest intermediateimage. Therefore, the folding mirrors are field mirrors. A double passregion where the radiation passes twice in opposite directions isthereby formed geometrically between the deflecting mirrors FM1, FM2 andthe concave mirror CM. A negative group NG having two negative lenses isarranged in a region with large marginal ray height near the concavemirror and coaxial with the concave mirror such that the radiationpasses twice in opposite directions through the negative group. Nooptical element is arranged between the negative group and the concavemirror.

The first optical element immediately adjacent to the object surface OSis a transparent plane-parallel plate PP arranged very close to theobject field. Following Table B gives the data for marginal ray heightMRH, chief ray height CRH, ray height ratio RHR and sub-aperture ratioSAR of the entry surface 1 and the exit surface 2 of the plate PP:

TABLE B MRH CRH RHR SAR 1 −39.77 −60.64 +1.5 +0.4 2 −41.50 −60.64 +1.5+0.4

The plate is associated with the manipulation device MAN allowing tovary the two-dimensional distribution of refractive index of the platematerial in response to electrical signals on a short time scale withhigh spatial resolution. The construction of the manipulator may bebased on the wire grid manipulator system disclosed in WO 2008/034636A2, which is incorporated herein by reference. A cooling system foractively cooling the transparent manipulator element may be provided toincrease the dynamics and allow for rapid changes of temperature.

Further, manipulation may be effected by relative displacement ofaspheric surfaces having complementary shape, such as shown, e.g. in EP0 851 304 B1. A pair of aspheres may be arranged immediately adjacent tothe object surface as a field element. Electro-optical manipulators mayalso be utilized. Further, a manipulator may be constructed to includecylinder lens elements rotatable relative to each other (see e.g. EP 0660 169 B1), and placed near a field surface, such as the objectsurface.

As in the first exemplary embodiment, three correction scenarios SC1,SC2 and SC3 have been simulated for comparison. In the first scenarioSC1 the substrate unevenness has been corrected by active manipulationof the wafer z-position and wafer tilt status, basically as explained inconnection with FIG. 3, in a conventional manner. In a second scenarioSC2 a number of active manipulations on optical elements as explainedabove have been performed in addition.

In a third scenario SC3 time-dependent lateral refractive indexinhomogeneities within the plane plate PP have been applied to optimizethe field curvature and other field aberrations of the projectionobjective dynamically during the scanning operation. The two-dimensional(spatial) distribution of refractive index may be described Zernikecoefficients Z2 to Z49, for example.

FIG. 11 shows a comparative diagram of aberrations induced by thequadratic unevenness (PV=45 nm) in the mentioned three scenarios SC1,SC2, SC3. The overall induced aberrations are decomposed incontributions described by Zernike coefficients, which are shown as thevalues on the abscissa of FIG. 11. The ordinate shows the scannedaberrations in nm. A significant improvement of the scanned aberrationsis immediately discernible for the case where the plate-like opticalelement PP is dynamically activated during the scanning operation toreduce aberrations. Both the defocus aberration (Z4) and the wavefronttilt aberration (Z2/3) could be reduced significantly both with respectto the standard scenario SC2 and even more reduced with respect to thefirst scenario SC1, where only an optimization of wafer position withrespect to axial position and tilt is used. Both defocus (Z4) andwavefront tilt (Z2/3) can be reduced by about 90% relative to thestandard scenario SC2, while even better improvement by about 93% isobtained relative to the pure wafer scenario SC1. For example, defocusaberration Z4 could be reduced from about 12 nm to about 0.8 nm bydynamic modification of reractive index distribution in the plate PP.Likewise, a significant improvement is obtained for astigmatism (Z5/6),which could be reduced by about 90% relative to the standard scenarioSC2. The parasitic astigmatism aberration Z2/3 is reduced from 0.7 nm(in scenario SC2) to 0.3 nm in scenario SC3.

These values indicate that the dynamic modifications of refractive powerdistribution in the field plate PP reduces significantly thecontribution of substrate surface unevenness to the focus budget, whereit is presently amongst the dominating contributions. A significantimprovement of process latitude in the lithographic process is therebyobtained. At the same time, the desired properties related to focuserrors can be relaxed with regard to other contributing effects, such aslens heating or the like.

The exemplary embodiments show that a dynamic change of imagingproperties, such as field curvature of the projection objective during ascanning operation may significantly improve the aberration level forthe entire exposure process in cases where the surface of the substrateto be exposed is not perfectly flat in the exposure area. However, thisis only one of many problems which may be solved or induced by providinga projection exposure system with a manipulation mechanism allowing atargeted change of the imaging properties of the projection objectiveduring a single scanning operation. Another problem which may beaddressed by the improved structure and functionality is the problemcommonly referred to as “reticle bending”.

In general, the projection objective is aligned with its optical axis inthe direction of gravity in typical exposure systems. The mask bearingthe pattern is then typically oriented in a horizontal plane,perpendicular to the optical axis. As a consequence the reticle (mask)may sag owing to the force of gravity, the sagging basically being afunction of the type of the reticle and of the mounting techniquesecuring the reticle in a reticle holder. In general, thetwo-dimensional deviation from a planar alignment of the pattern may notbe known a priori and may be difficult to determine. As a consequence ofthe sagging, individual locations on the reticle which are to be imagedmay be displaced from their desired position (given at a perfectlyplanar reticle) in a way that cannot be completely predicted a priori,the direction and length of this displacement generally being a functionof the location on the reticle. A further cause for possible sagging ofthe reticle is the direct influence of the mounting technique on theshape of the mounted reticle. In general, forces and moments caused bybearings and/or clamps acting on the reticle may contribute to a complexdeformation status of the reticle during the exposure. These influencesmay not be fully known a priori and can differ from reticle to reticle,but they can also be identical for classes of reticles.

Problems resulting from reticle bending have been frequently addressedin various ways, such as demonstrated for example in WO 2006/01300 A2 bythe applicant or in US 2003/0133087 A1.

The problems of aberrations caused by reticle bending may be addressedin a dynamic fashion with exemplary embodiments of the exposureapparatus configured to dynamically change the imaging properties of theprojection objective during a scanning operation.

FIG. 12 shows qualitatively deformations of a multiply bend reticlesurface in a perspective view. If the reticle is held by a support on aframe, the deformation may assume a substantially parabolic shape, asshown, for example, in WO 2006/013100 A2. If the reticle is mounted by aclamping technique engaging at three or more points in a peripheralregion of the reticle, a saddle-shaped deformation may result, which issuperimposed on the gravity-induced deformation. Such saddle-shapeddeformation is shown in FIG. 12 for a reticle that is clamped at fourcorner positions. The reticle bending may result in a displacement ofthe center region of the reticle relative to the edge region in theorder of one or more tenth of a micrometer. It may be possible toaccount for a systematic reticle bending effect by counteractivemeasures during adjustment of a projection objective such that theprojection objective has a defined non-zero field curvature whichglobally accounts for a certain amount of reticle bending. However,non-systematic contributions caused by using different reticles and/orcaused by hardly predictable temperature induced deformations during theoperation may not be fully accounted for in advance by correspondingadjustment of the projection objective. Those non-predictable effectsmay contribute significantly to the focus budget and it may not bepossible to fully account for the corresponding aberration effects bystandard manipulations. However, active manipulation of at least onefield element in a dynamic fashion during a scanning operation maysignificantly improve the aberration level towards smaller values.

FIG. 13 shows a comparative view of two correction scenarios. The figureshows the residual aberrations after adjustment of the manipulators. Thecolumns show the residual errors (scanned aberrations) in a standardscenario SC2 for an anamophotic (saddle-shaped) deformation of thereticle with 400 nm PV-deformation at the reticle center. In comparison,the solid line SC3 shows respective values in a correction scenarioincluding targeted deformations of the two concave mirrors CM1, CM2(field mirrors) of the exemplary embodiment of FIG. 7. The defocus valueZ4 amounts to about 7 nm in a system without correction. The defocuserror can be reduced to about 3.5 nm in the standard scenario SC2.Additional targeted deformation of the concave reflective surfaces ofmirrors CM1, CM2 can further reduce the defocus by about an order ofmagnitude to about 0.3 nm. At the same time, the parasitic errorsinduced by this correction are generally small, such as below 0.5 nm asshown.

A similar study has been performed for the second exemplary embodimentemploying a transparent field PP element coupled with a manipulatorallowing for a change in the two-dimensional distribution of refractiveindex within the plate. FIG. 14 shows a comparative view of threecorrection scenarios. Employing the standard correction allows to reduceresidual errors of the focus (Z4) from about 7 nm to about 6 nmEmploying the two-dimensional manipulator acting on a field elementclose to the object surface allows for significantly reducing thoseerrors. Specifically, the defocus aberration may be reduced from about 6nm to about 0.4 nm without introducing parasitic field aberrations abovea critical level. In addition, the tilt error Z2/3 could be reduced fromabout 16.8 nm in scenario SC1 (Wafer manipulations only) to about 0.6 nmwith the 2D-manipulator arranged close to the object surface.

FIG. 15 shows an example of another option to manipulate the opticalproperties of a projection objective during scanning by bending one ormore reflective surfaces of mirrors arranged optically close to a fieldsurface. FIG. 15 shows a detail of a meridional section through aprojection objective where all optical elements have the samespecification as described in connection with FIG. 10A, 10B. Therefore,the specification of the optical system shown partly in FIG. 15 is thesame as given in Tables 10, 10A. As described above, the secondobjective part OP2 includes a concave mirror CM situated close to thesecond pupil surface P2 of the projection objective, which is situatedoptically between the first intermediate image IMI1 (generated by therefractive first objective part) and the second intermediate image IMI2,which is finally imaged by the third, refractive objective part to formthe image in the image surface. The first folding mirror FM1 is arrangedoptically close to the first intermediate image and reflects radiationprovided by the first objective part OP1 towards the concave mirror CM.The second folding mirror FM2, arranged at 90° relative to the firstfolding mirror FM1, is arranged optically close to the secondintermediate image IMI2 and reflects radiation coming from the concavemirror CM towards the image surface. The reflective surfaces of both thefirst and the second folding mirror are substantially flat (planar) intheir nominal state of operation wherein they have no optical power andtheir only function is to deflect radiation incident thereon. Table Cbelow gives the data for the marginal ray height MRH, chief ray heightCRH, ray height ratio RHR and sub-aperture ratio SAR of the firstfolding mirror FM1 and the second folding mirror FM2, respectively. Itis easily seen particularly from the sub-aperture ratio that bothfolding mirrors are situated very close to a field surface (SAR close tozero).

TABLE C MRH CRH RHR SAR FM1 +2.37 +131.27 +55.4 +0.018 FM2 −7.67 −112.09+14.6 +0.064

In this exemplary embodiment, the portions of the substrates of thefolding mirrors carrying the reflective mirror coating are flexible to alimited extent such that the reflective surface area can be bend inresponse to external forces to a certain extent. A number of actuators(represented by arrows) of a first mirror manipulator MM1 associatedwith the first folding mirror FM1 and a second mirror manipulator MM2associated with the second folding mirror FM2 are operatively coupled tothe backside of the flexible portion of the respective mirrors. Theactuators are controlled by a mirror control unit MCU, which may be anintegral part of a central control unit of the projection exposureapparatus. The mirror control unit is connected to receive signalsrepresenting a desired deformation, or an absence of deformation, of therespective mirror surfaces. Each mirror manipulator is configured tobend the associated reflective surface either in one dimension only (forexample cylindrical shape of the mirror surface) or in two dimensions(for example substantially concave or convex spherical or asphericalmirror surface shape).

The bending manipulation of one or both folding mirrors ina an inwardand/or an outward direction allows a variety of degrees of freedom tomanipulate the optical performance of the projection objectiveparticularly with respect to field aberrations. The mirror surfaces canbe deformed in a time-dependent manner during a scanning operation toreduce undesired aberrations, such as aberrations caused by anunevenness of the substrate surface. Since there are two manipulatorswhich may be operated independent of each other, different aberrationsmay be corrected independently from each other. Further, an additionalmanipulation range may be obtained in cases where manipulation of asingle mirror would not be sufficient to provide the range ofmanipulation involved to compensate certain aberrations. The effectsobtainable by manipulating the folding mirrors may be similar to thoseobtainable by manipulating concave mirrors arranged close tointermediate images such as described in connection with FIG. 7A, 7B.Therefore, reference is made to that description. One or more foldingmirrors having an actively deformable reflective surface may be providedin addition to other manipulators, such as the transparent manipulatorMAN formed by the plane plate PP in FIG. 10B to provide further degreesof freedom. Other exemplary embodiments do not have a plane plates whichcan be manipulated in the described manner.

It has been shown with various exemplary embodiments that a real-timecorrection of field aberrations, such as field curvature, during ascanning operation in a scanning projection exposure apparatus maysignificantly reduce imaging aberrations which may be induced bydeviations of the substrate surface and/or of the surface of the patternfrom the ideal planar shape. Specifically, negative effects of wafersurface unevenness and/or reticle bending in scanning exposure systemsmay be significantly reduced.

The above description of exemplary embodiments has been given by way ofexample. From the disclosure given, those skilled in the art will notonly understand the present disclosure and its attendant advantages, butwill also find apparent various changes and modifications to thestructures and methods disclosed. It is sought, therefore, to cover allchanges and modifications as fall within the spirit and scope of thedisclosure, as defined by the appended claims, and equivalents thereof

TABLE 7 INDEX 1/2 FREEE SURFACE RADIUS THICKNESS MATERAL 193.37 nmDIAMETER OBJECT 0.000000 30.000000 1.000000 63.50 1 155.500468 47.980384SIO2 1.560188 82.82 2 −333.324258 1.000851 1.000000 82.26 3 240.65094910.194017 SIO2 1.560188 79.90 4 106.801800 20.365766 1.000000 74.67 5124.630972 34.680113 SIO2 1.560188 79.59 6 468.398192 11.981603 1.00000077.78 7 380.553803 26.909838 SIO2 1.560188 77.24 8 −171.820449 19.3369731.000000 77.05 9 −3157.552773 18.733166 SIO2 1.560188 63.69 10−201.010840 2.828885 1.000000 61.54 11 0.000000 10.000000 SIO2 1.56018854.63 12 0.000000 14.057031 1.000000 51.30 13 −675.142412 17.151908 SIO21.560188 45.97 14 −177.431040 17.484199 1.000000 49.62 15 0.00000010.000000 SIO2 1.560188 57.65 16 0.000000 48.376949 1.000000 59.68 17−339.503853 22.085648 SIO2 1.560188 72.90 18 −153.734447 9.4263431.000000 75.79 19 −133.551892 10.000178 SIO2 1.560188 76.55 20−159.012061 260.928174 1.000000 80.56 21 −186.269426 −223.122910−1.000000 159.97 REFL 22 171.856468 290.241437 1.000000 137.56 REFL 23418.208640 33.119326 SIO2 1.560188 109.84 24 −764.923828 24.9917121.000000 109.22 25 −933.573206 23.101710 SIO2 1.560188 104.46 261486.991752 3.727360 1.000000 103.09 27 264.108066 15.536565 SIO21.560188 94.14 28 124.187755 40.232391 1.000000 84.09 29 −905.19855811.197639 SIO2 1.560188 83.89 30 131.424652 22.232119 1.000000 82.63 31288.907138 18.371287 SIO2 1.560188 85.15 32 1443.815086 26.0393701.000000 87.98 33 −219.723661 10.212957 SIO2 1.560188 90.08 34−505.370348 1.495833 1.000000 104.53 35 602.513212 45.614756 SIO21.560188 113.36 36 −381.370078 0.999817 1.000000 124.37 37 −3646.79354062.876806 SIO2 1.560188 133.45 38 −186.442382 0.999658 1.000000 138.7939 803.321916 47.355581 SIO2 1.560188 156.65 40 −403.820101 0.9993751.000000 158.05 41 464.394742 43.310049 SIO2 1.560188 156.92 42−28298.847889 5.923544 1.000000 155.75 Aper.St. 0.000000 −4.9244371.000000 153.99 44 452.887984 57.784133 SIO2 1.560188 151.45 45−566.954376 1.000000 1.000000 149.33 46 114.038890 60.833075 SIO21.560188 99.52 47 1045.400093 1.000000 1.000000 90.06 48 61.10542743.354396 SIO2 1.560188 51.24 49 0.000000 3.100000 H2O 1.436182 24.42IMAGE 0.000000 0.000000 H2O 1.436182 15.88 NA = 1.35, β = −0.25, λ = 193nm, image field size 26 mm * 5.5 mm.

TABLE 7A Aspherical Constants SRF 2 7 10 21 K 0.000000 0.000000 0.000000−2.179800 C1 6.25615673E−08 −3.91970883E−07 −1.05740650E−07−3.48837141E−08 C2 3.98291586E−12 2.09559113E−11 4.58061705E−112.87484829E−13 C3 −9.45171648E−16 9.82281688E−16 −9.11506463E−15−9.16227915E−18 C4 1.33506075E−19 −1.66737901E−20 3.32932453E−181.68127084E−22 C5 −1.30131298E−23 −4.32368642E−23 −7.84340820E−22−3.70862206E−27 C6 8.34436130E−28 5.15009335E−27 1.06485056E−254.91461297E−32 C7 −2.58453411E−32 −2.09824486E−31 −6.44096726E−30−4.06294509E−37 C8 0.00000000E+00 0.00000000E+00 0.00000000E+000.00000000E+00 C9 0.00000000E+00 0.00000000E+00 0.00000000E+000.00000000E+00 SRF 22 26 29 31 K −0.680600 0.000000 0.000000 0.000000 C17.08665406E−09 −2.25360263E−07 2.93338485E−08 4.63356893E−09 C21.05796279E−13 1.35102880E−11 3.99085713E−13 −4.99736404E−12 C31.49167226E−18 3.79578877E−17 −6.05050026E−17 4.88963742E−16 C41.62571301E−23 −7.42121392E−20 1.25522927E−19 −1.48446943E−19 C54.83415117E−28 6.11793938E−24 −1.87742919E−23 1.97755334E−23 C6−3.21154060E−33 −2.34729324E−28 1.07269082E−27 −1.91898457E−27 C71.42531822E−37 3.43409149E−33 −3.21830574E−32 8.50123020E−32 C80.00000000E+00 0.00000000E+00 0.00000000E+00 0.00000000E+00 C90.00000000E+00 0.00000000E+00 0.00000000E+00 0.00000000E+00 SRF 34 35 3739 K 0.000000 0.000000 0.000000 0.000000 C1 1.03668480E−07−4.70359496E−08 −1.94534158E−10 −5.11706590E−08 C2 2.75801089E−124.15939963E−12 −3.07266701E−12 1.60140405E−12 C3 −3.27807145E−16−3.90892000E−16 1.39267169E−16 1.10341691E−17 C4 −2.15301048E−201.64438627E−20 −1.59434604E−21 −7.04763579E−22 C5 1.08976725E−24−1.80866983E−25 −5.98715796E−26 −3.28833297E−26 C6 6.73532870E−29−1.85786454E−29 2.24005496E−30 1.48518840E−30 C7 −2.25221802E−336.59409175E−34 0.00000000E+00 −1.52353104E−35 C8 0.00000000E+000.00000000E+00 0.00000000E+00 0.00000000E+00 C9 0.00000000E+000.00000000E+00 0.00000000E+00 0.00000000E+00 SRF 42 45 47 K 0.0000000.000000 0.000000 C1 −5.79976918E−08 2.42988807E−08 3.85557367E−08 C22.83727775E−12 −1.85555835E−12 7.25233559E−12 C3 −1.20786196E−17−2.74974282E−17 −1.15590543E−15 C4 −3.22879626E−21 6.44628653E−211.27817902E−19 C5 1.90525889E−25 −2.71099457E−25 −1.08124491E−23 C6−5.24966838E−30 5.29432933E−30 5.72390431E−28 C7 5.62271368E−35−4.24451451E−35 −1.24244050E−32 C8 0.00000000E+00 0.00000000E+000.00000000E+00 C9 0.00000000E+00 0.00000000E+00 0.00000000E+00

TABLE 10 INDEX 1/2 FREE SURFACE RADIUS THICKNESS MATERIAL 193.31 nmDIAMETER OBJECT 0.000000 63.754200 1.000000 60.64 1 0.000000 8.000000SILUV 1.560482 80.57 2 0.000000 56.000000 1.000000 82.16 3 930.92392252.000000 SILUV 1.560482 102.42 4 −256.201417 1.000000 1.000000 106.20 5164.802556 35.773100 SILUV 1.560482 106.58 6 341.545138 15.7479001.000000 103.06 7 147.535157 56.488000 SILUV 1.560482 94.36 8−647.942934 4.145000 1.000000 87.10 9 −536.071478 18.297900 SILUV1.560482 84.98 10 180.585020 1.000000 1.000000 69.07 11 82.24709628.431900 SILUV 1.560482 62.83 12 121.636868 21.482876 1.000000 55.13 130.000000 10.000000 SILUV 1.560482 49.65 14 0.000000 35.037652 1.00000045.92 15 −89.601791 44.878000 SILUV 1.560482 49.19 16 −203.30835749.953200 1.000000 70.60 17 −333.934057 37.672400 SILUV 1.560482 95.5118 −153.471299 1.000000 1.000000 101.95 19 −588.427923 47.008300 SILUV1.560482 109.74 20 −177.569099 1.000000 1.000000 113.61 21 1289.63545232.747800 SILUV 1.560482 110.26 22 −409.790925 1.000000 1.000000 109.2723 196.979548 36.289500 SILUV 1.560482 97.10 24 2948.592605 72.0000001.000000 91.85 25 0.000000 −204.306500 −1.000000 88.41 REFL 26120.965260 −15.000000 SILUV −1.560482 67.60 27 177.749728 −28.181900−1.000000 76.71 28 106.065668 −18.000000 SILUV −1.560482 81.66 29323.567743 −34.983200 −1.000000 112.46 30 165.900097 34.983200 1.000000118.94 REFL 31 323.567743 18.000000 SILUV 1.560482 114.42 32 106.06566828.181900 1.000000 87.43 33 177.749728 15.000000 SILUV 1.560482 86.09 34120.965260 204.306500 1.000000 77.89 35 0.000000 −72.000000 −1.00000096.41 REFL 36 462.513697 24.493400 SILUV −1.560482 90.96 37 196.771640−1.000000 −1.000000 94.59 38 −996.046057 −27.579900 SILUV −1.560482104.05 39 480.084349 −1.000000 −1.000000 105.63 40 −260.478322−35.771400 SILUV −1.560482 109.79 41 −3444.700345 −1.000000 −1.000000108.53 42 −189.044457 −50.000000 SILUV −1.560482 104.52 43 −630.985131−43.198700 −1.000000 96.24 44 675.856906 −10.000000 SILUV −1.56048285.17 45 −117.005373 −46.536000 −1.000000 77.36 46 214.318111 −10.000000SILUV −1.560482 78.01 47 −191.854301 −23.664400 −1.000000 90.88 481573.576031 −31.506600 SILUV −1.560482 92.88 49 214.330939 −1.000000−1.000000 99.07 50 −322.859172 −33.185600 SILUV −1.560482 131.18 51−1112.917245 −10.017200 −1.000000 133.09 52 −2810.857827 −22.000000SILUV −1.560482 134.79 53 −920.532878 −42.079900 −1.000000 143.55 54707.503574 −62.025500 SILUV −1.560482 144.85 55 238.350224 −1.000000−1.000000 155.97 56 −17926.557240 −62.132800 SILUV −1.560482 176.77 57336.363925 −2.000000 −1.000000 178.83 58 0.000000 −10.000000 SILUV−1.560482 178.34 59 0.000000 −51.180119 −1.000000 178.29 60 0.00000048.529765 −1.000000 178.13 61 −303.574400 −68.224400 SILUV −1.560482178.47 62 −19950.680601 −7.986643 −1.000000 176.32 63 −182.034245−77.612200 SILUV −1.560482 149.97 64 −459.526735 −1.000000 −1.000000140.24 65 −130.446554 −49.999900 SILUV −1.560482 104.87 66 −393.038792−1.000000 −1.000000 90.82 67 −76.745086 −43.335100 SILUV −1.560482 62.0868 0.000000 −1.000000 H2OV −1.435876 43.69 69 0.000000 13.000000 SILUV−1.560482 41.39 70 0.000000 −3.000396 H2OV −1.435876 21.49 IMAGE0.000000 0.000000 H2OV −1.435876 15.16 NA = 1.35, β = −0.25, λ = 193 nm,image field size 26 mm * 5.5 mm.

TABLE 10A Aspheric Constants SURFACE 9 17 24 43 K 0.000000 0.0000000.000000 0.000000 C1 −6.15279430E−08 −7.45094812E−09 2.39757847E−08−1.54444107E−08 C2 9.07235846E−12 −4.96869140E−13 −3.99268761E−134.80555171E−13 C3 −7.62016156E−16 −1.80457306E−17 7.58714696E−18−6.81317137E−18 C4 2.11604502E−20 1.21427645E−21 −4.07907824E−23−4.66659168E−22 C5 2.96357585E−24 5.49372107E−27 −1.80719028E−261.24273210E−25 C6 −3.07533659E−28 −5.29980296E−30 1.56076846E−30−9.55749906E−30 C7 9.06861666E−33 1.87024195E−34 −4.46650154E−352.28466028E−34 C8 0.00000000E+00 0.00000000E+00 0.00000000E+000.00000000E+00 C9 0.00000000E+00 0.00000000E+00 0.00000000E+000.00000000E+00 SURFACE 47 48 51 53 K 0.000000 0.000000 0.000000 0.000000C1 8.90249656E−08 2.76818942E−08 1.78760001E−08 −3.73813506E−08 C2−4.45589909E−12 −1.65762708E−12 −3.78510161E−13 −8.19572990E−13 C33.52161972E−16 1.59654730E−17 −9.90977073E−17 1.19684623E−16 C4−2.12193214E−20 −5.50850519E−21 4.44216686E−21 −2.57891394E−21 C52.64252109E−25 5.52122259E−25 −1.56666224E−25 −3.00853670E−26 C61.24505795E−28 −1.35900754E−28 6.86877450E−30 1.83635093E−30 C7−6.14481041E−33 8.92763147E−33 −1.16755871E−34 −2.23365868E−35 C80.00000000E+00 0.00000000E+00 0.00000000E+00 0.00000000E+00 C90.00000000E+00 0.00000000E+00 0.00000000E+00 0.00000000E+00 SURFACE 5464 66 K 0.000000 0.000000 0.000000 C1 1.72819429E−08 3.26401527E−08−6.53868478E−08 C2 −6.69013106E−13 −2.29140025E−12 −1.10187961E−13 C31.37958358E−17 1.41955877E−16 −2.52289121E−16 C4 −5.77392139E−22−6.67154130E−21 2.78201377E−20 C5 1.73608231E−26 2.03231717E−25−3.07320360E−24 C6 −3.71846793E−31 −3.61901511E−30 1.72608145E−28 C76.53070350E−36 2.87175589E−35 −5.63187727E−33 C8 0.00000000E+000.00000000E+00 0.00000000E+00 C9 0.00000000E+00 0.00000000E+000.00000000E+00

What is claimed is:
 1. A method, comprising: exposing an exposure areaof a radiation-sensitive substrate arranged in an image surface of aprojection objective with at least one image of a pattern of a maskarranged in an object surface of the projection objective in a scanningoperation, the scanning operation comprising moving the mask relative toan effective object field of the projection objective in a firstscanning direction and simultaneously moving the radiation-sensitivesubstrate relative to an effective image field of the projectionobjective in a second scanning direction; and actively changing imagingproperties of the projection objective during the scanning operationaccording to a time profile to: a) dynamically vary a field curvature ofthe projection objective during the scanning operation; and b)dynamically vary a distortion of the projection objective during thescanning operation, wherein: the projection objective comprises aplurality of mirrors including a folding mirror and a concave mirror;the folding mirror comprises a reflective surface arranged in aprojection beam path of the projection objective at a location that isoptically close to a field surface of the projection objective; theconcave mirror comprises a reflective surface arranged in the projectionbeam path; and varying the field curvature and the distortion of theprojection objective comprises adjusting a surface profile of areflective surface of a first mirror of the projection objective from asingle radius of curvature having a first value in an optically usedarea prior to adjusting to a single radius of curvature having a secondvalue in the optically used area after the adjusting, to change anoptical effect caused by the mirror in a spatially resolved manner,wherein the adjusting results in a single radius of curvature after theadjusting, the second value being different from the first value.
 2. Themethod of claim 1, comprising bending the folding mirror.
 3. The methodof claim 1, wherein the first mirror is the folding mirror.
 4. Themethod of claim 1, wherein the projection objective further comprises afield element, and the method comprises manipulating the folding mirrorand the field element independently of each other.
 5. The method ofclaim 1, wherein the field curvature is changed at a rate that isbetween 0.5% and 50% of a depth of focus of the projection objectivewithin a one millisecond time interval.
 6. The method of claim 1,wherein a direction of change of at least one aberration is changed oneor more times during a single scanning operation.
 7. The method of claim1, wherein imaging properties are changed with a frequency of at least20 Hz.
 8. The method of claim 1, further comprising: generating datarepresenting a surface profile of the radiation-sensitive substrate inthe exposure area of the radiation-sensitive substrate; generatingmanipulator control signals based on the data; and driving at least onemanipulation device of the projection objective in response to themanipulator control signals to dynamically adapt the imaging propertiesof the projection objective to reduce imaging aberrations caused by thesurface profile in the exposure area of the radiation-sensitivesubstrate.
 9. The method of claim 1, further comprising: generating datarepresenting a surface profile of the mask in the exposure area of themask; generating manipulator control signals based on the data; anddriving at least one manipulation device of the projection objective inresponse to the manipulator control signals to dynamically adapt theimaging properties of the projection objective to reduce imagingaberrations caused by the surface profile in the exposure area of themask.
 10. The method of claim 1, wherein: the projection objectivefurther comprises a field element that is a transparent optical elementin the projection beam path; the transparent optical element has anoptical surface arranged in the projection beam path of the projectionobjective optically close to a field surface of the projectionobjective; and the method comprises changing a spatial distribution ofrefractive power in an optically used area of the transparent opticalelement.
 11. A method, comprising: a) exposing an exposure area of aradiation-sensitive substrate in an image surface of a projectionobjective with at least one image of a pattern of a mask arranged in anobject surface of the projection objective while moving the maskrelative to an effective object field of the projection objective in afirst direction and simultaneously moving the radiation-sensitivesubstrate relative to an effective image field of the projectionobjective in a second direction; and b) during a), dynamically vary afield curvature of the projection objective and dynamically vary adistortion of the projection objective during the scanning operation,wherein: the projection objective comprises a plurality of mirrorsincluding a folding mirror and a concave mirror; the folding mirrorcomprises a reflective surface arranged in a projection beam path of theprojection objective at a location that is optically close to a fieldsurface of the projection objective; the concave mirror comprises areflective surface arranged in the projection beam path; and varying thefield curvature and the distortion of the projection objective comprisesadjusting a surface profile of a reflective surface of a first mirror ofthe projection objective from a single radius of curvature having afirst value in an optically used area prior to adjusting to a singleradius of curvature having a second value in the optically used areaafter the adjusting, to change an optical effect caused by the firstmirror in a spatially resolved manner, wherein the adjusting results ina single radius of curvature after the adjusting, the second value beingdifferent from the first value.
 12. The method of claim 11, comprisingbending the folding mirror.
 13. The method of claim 11, wherein thefirst mirror is the folding mirror.
 14. An apparatus, comprising: anillumination system configured to produce illumination radiationincident on an object bearing a pattern; a projection objectiveconfigured to project an image of the pattern onto a radiation-sensitivesubstrate; a scanning system configured to, during a scanning operation,move the object relative to an effective object field of the projectionobjective in a first direction and to simultaneously move theradiation-sensitive substrate relative to the effective image field ofthe projection objective in a second direction; and a control systemconfigured to, during the scanning operation, dynamically vary a fieldcurvature of the projection objective and dynamically vary a distortionof the projection objective during the scanning operation, wherein: theprojection objective comprises a plurality of mirrors including afolding mirror and a concave mirror; the folding mirror comprises areflective surface arranged in a projection beam path of the projectionobjective at a location that is optically close to a field surface ofthe projection objective; the concave mirror comprises a reflectivesurface arranged in the projection beam path; and the projectionobjective comprises a device configured to adjust a surface profile of areflective surface of a first mirror of the projection objective from asingle radius of curvature having a first value in an optically usedarea prior to adjusting to a single radius of curvature having a secondvalue in the optically used area after the adjusting, to change anoptical effect caused by the first mirror in a spatially resolvedmanner, wherein the adjusting results in a single radius of curvatureafter the adjusting, the second value being different from the firstvalue, thereby varying the field curvature and the distortion of theprojection objective.
 15. The apparatus of claim 14, wherein the deviceis configured to bend the folding mirror.
 16. The apparatus of claim 14,wherein the first mirror is the folding mirror.
 17. The apparatus ofclaim 14, wherein the projection objective comprises: a first objectivepart configured to image the pattern provided in the object surface intoa first intermediate image; a second objective part configured to imagethe first intermediate image into a second intermediate image, thesecond objective part comprising the concave mirror which is situated ata second pupil surface; a third objective part configured to image thesecond intermediate image onto the image surface, wherein: the foldingmirror is arranged optically close to the first intermediate image suchthat the folding mirror can reflect radiation coming from the objectsurface in the direction of the concave mirror; and the projectionobjective comprises a further folding mirror arranged optically close tothe second intermediate image such that the further folding mirror canreflect radiation coming from the concave mirror in the direction of theimage surface.
 18. The apparatus of claim 17, wherein each of thefolding mirrors is associated with a manipulation device, and themanipulation devices are configured to change the surface profiles ofthe folding mirrors in a prescribed coordinated manner independently ofeach other.
 19. The apparatus of claim 14, further comprising a deviceconfigured to determine a topography of the radiation-sensitivesubstrate prior to varying the field curvature of the projectionobjective.
 20. The apparatus of claim 19, wherein determining atopography of the radiation-sensitive substrate comprises determining anamount of deviation from a planar reference surface, the planarreference surface being perpendicular to an optical axis of theprojection objective.